the quantum story This page intentionally left blank THE Quantum Story a history in 40 moments jim baggott 1 1 Great Clarendon Street, Oxford ox2 6dp Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide in Oxford New York Auckland Cape Town Dar es Salaam Hong Kong Karachi Kuala Lumpur Madrid Melbourne Mexico City Nairobi New Delhi Shanghai Taipei Toronto With offi ces in Argentina Austria Brazil Chile Czech Republic France Greece Guatemala Hungary Italy Japan Poland Portugal Singapore South Korea Switzerland Thailand Turkey Ukraine Vietnam Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries Published in the United States by Oxford University Press Inc., New York © Jim Baggott 2011 The moral rights of the authors have been asserted Database right Oxford University Press (maker) First published 2011 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, or under terms agreed with the appropriate reprographics rights organization. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above You must not circulate this book in any other binding or cover and you must impose the same condition on any acquirer British Library Cataloguing in Publication Data Data available Library of Congress Cataloging in Publication Data Data available Typeset by SPI Publisher Services, Pondicherry, India Printed in Great Britain on acid-free paper by Clays Ltd, St Ives plc ISBN 978–0–19–956684–6 10987654321 For MSS, because every student deserves at least one great teacher This page intentionally left blank CONTENTS Preface xiii Prologue: Stormclouds 1 London, April 1900 PART I: QUANTUM OF ACTION 1 The Most Strenuous Work of My Life 7 Berlin, December 1900 2 Annus Mirabilis 17 Bern, March 1905 3 A Little Bit of Reality 25 Manchester, April 1913 4 la Comédie Française 34 Paris, September 1923 5 A Strangely Beautiful Interior 43 Helgoland, June 1925 6 The Self-rotating Electron 51 Leiden, November 1925 7 A Late Erotic Outburst 60 Swiss Alps, Christmas 1925 PART II: QUANTUM INTERPRETATION 8 Ghost Field 71 Oxford, August 1926 9 All This Damned Quantum Jumping 79 Copenhagen, October 1926 vii the quantum story 10 The Uncertainty Principle 87 Copenhagen, February 1927 11 The ‘Kopenhagener Geist’ 95 Copenhagen, June 1927 12 There is No Quantum World 103 Lake Como, September 1927 PART III: QUANTUM DEBATE 13 The Debate Commences 115 Brussels, October 1927 14 An Absolute Wonder 126 Cambridge, Christmas 1927 15 The Photon Box 133 Brussels, October 1930 16 A Bolt from the Blue 141 Princeton, May 1935 17 The Paradox of Schrödinger’s Cat 149 Oxford, August 1935 Interlude: The First War of Physics 159 Christmas 1938–August 1945 PART IV: QUANTUM FIELDS 18 Shelter Island 171 Long Island, June 1947 19 Pictorial Semi-vision Thing 181 New York, January 1949 20 A Beautiful Idea 193 Princeton, February 1954 21 Some Strangeness in the Proportion 204 Rochester, August 1960 22 Three Quarks for Muster Mark! 214 New York, March 1963 viii contents 23 The ‘God Particle’ 225 Cambridge, Massachusetts, Autumn 1967 PART V: QUANTUM PARTICLES 24 Deep Inelastic Scattering 237 Stanford, August 1968 25 Of Charm and Weak Neutral Currents 247 Harvard, February 1970 26 The Magic of Colour 256 Princeton/Harvard, April 1973 27 The November Revolution 265 Long Island/Stanford, November 1974 28 Intermediate Vector Bosons 275 Geneva, January/June 1983 29 The Standard Model 285 Geneva, September 2003 PART VI: QUANTUM REALITY 30 Hidden Variables 297 Princeton, Spring 1951 31 Bertlmann’s Socks 306 Boston, September 1964 32 The Aspect Experiments 318 Paris, September 1982 33 The Quantum Eraser 328 Baltimore, January 1999 34 Lab Cats 339 Stony Brook/Delft, July 2000 35 The Persistent Illusion 349 Vienna, December 2006 PART VII: QUANTUM COSMOLOGY 36 The Wavefunction of the Universe 361 Princeton, July 1966 ix the quantum story 37 Hawking Radiation 372 Oxford, February 1974 38 The First Superstring Revolution 381 Aspen, August 1984 39 Quanta of Space and Time 391 Santa Barbara, February 1986 40 Crisis? What Crisis? 399 Durham, Summer 1994 Epilogue: A Quantum of Solace? 407 Geneva, March 2010 Notes and Sources 411 Bibliography 441 Plate Acknowledgements 448 Index 451 x ABOUT THE AUTHOR Jim Baggott is an award-winning science writer. A former academic sci- entist, he now works as an independent business consultant but main- tains a broad interest in science, philosophy, and history, and continues to write on these subjects in his spare time. His previous books have been widely acclaimed and include: Atomic: The First War of Physics and the Secret History of the Atom Bomb 1939–49 (Icon Books, 2009); A Beginner’s Guide to Reality (Penguin, 2005); Beyond Measure: Modern Physics, Philosophy and the Meaning of Quantum The- ory (Oxford University Press, 2004); Perfect Symmetry: The Accidental Discovery of Buckminsterfullerene (Oxford University Press, 1994); and The Meaning of Quantum Theory: A Guide for Students of Chemistry and Physics (Oxford University Press, 1992). xi This page intentionally left blank PREFACE The last century was defi ned by physics. From the minds of the world’s leading physicists there fl owed a river of ideas that would transport man- kind to the very pinnacle of wonder and to the very depths of despair. This was a century that began with the certainties of absolute knowledge and ended with the knowledge of absolute uncertainty. It was a century in which physicists developed theories that would deny us the possibility that we can ever properly comprehend the nature of physical reality. It was also a century in which they built weapons with the capacity utterly to destroy this reality. Almost everything we think we know about the nature of our world comes from one theory of physics. This theory was discovered and refi ned in the fi rst thirty years of the twentieth century and went on to become quite simply the most successful theory of physics ever devised. Its concepts underpin much of the twenty-fi rst century technology that we have learned to take for granted. But this success has come at a price, for it has at the same time com- pletely undermined our ability to make sense of the world at the level of its most fundamental constituents. Rejecting the elements of uncertainty and chance implied by this new theory, Albert Einstein once famously declared that ‘God does not play dice’. Niels Bohr claimed that anybody who is not shocked by the theory has not understood it. The charismatic American physicist Richard Fey- nman went further: he claimed that nobody understands it. To anyone tutored in the language and the logic of classical physics, this theory is at once mathematically challenging, maddeningly bizarre, and breathtak- ingly beautiful. This is quantum theory, and this book tells its story. xiii the quantum story If we fi x on Max Planck’s discovery of his ‘quantum of action’ in Decem- ber 1900 as the historical origin of the quantum theory, then as I write this theory is 110 years old. Time enough, you would have thought, for physi- cists to get to grips with it and understand what it means. Time enough to come to terms with what quantum theory has to say about chance and causality and the nature of physical reality. And yet, if anything, the sense of shock has increased, not diminished, with the passing of time. While nobody really understands how quantum theory actually works, the rules of its application are unquestioned and the accuracy and preci- sion of its predictions are unsurpassed in the entire history of science. Although heated debate continues about how quantum theory should be interpreted, there can be no debate about whether or not the theory is fundamentally correct. For more than four hundred years we nurtured the belief (should that, perhaps, be faith?) that evidence-based investigation meeting scientifi c standards of rigour would reveal the true mechanisms of nature. And yet when the mechanisms of nature were revealed to be quantum mech- anisms, the worlds of science and philosophy were set on a collision course. Instead of truth and comprehension, we got deeply unsettling questions about what we can ever hope to know about the world. Quan- tum theory pushed us to the edge of an epistemological precipice. Since the mid-1920s we have lived in fear of stepping over the edge. This book is a celebration of this wonderful yet wholly disconcerting theory, from its birth in the porcelain furnaces used to study black-body radiation in 1900 to the promise of stimulating new quantum phenom- ena to be revealed by CERN’s Large Hadron Collider, over a century later. It is a history told in forty ‘moments’, signifi cant moments of truth or turning points in the theory’s development. This history takes us on a long journey. Part I deals with Planck’s dis- covery in 1900 and traces the development of early quantum theory through Einstein’s light-quantum hypothesis, Bohr’s quantum theory of the atom, Louis de Broglie’s dual wave–particle hypothesis, Werner Heisenberg’s matrix mechanics, the puzzling phenomenon of electron spin, and Wolfgang Pauli’s exclusion principle. This section concludes with Erwin Schrödinger’s ‘late erotic outburst’, which led him to wave mechanics in December 1925. xiv preface In Part II, the book traces the development of the Copenhagen inter- pretation of quantum theory.